This invention relates to titanium alloy castings. In particular, it relates to a method for improving the microstructure of titanium alloy castings.
Titanium and titanium alloys are extremely valuable in uses where light weight and high strength-to-weight ratio are important. The casting of titanium and titanium alloys presents a special problem due to the high reactivity of the material in the molten state. This requires special melting, mold-making practices, and equipment to prevent alloy contamination. At the same time, titanium castings present certain advantages when compared to castings of other metals. The microstructure of as-cast titanium is desirable for many mechanical properties. It has good creep resistance, fatigue crack growth resistance, fracture resistance, and tensile strength. Titanium alloy castings also readily lend themselves to full densification by hot isostatic pressing (HIP) because they dissolve their own oxides at high temperatures allowing a complete closure of all non-surface-connected, i.e., non-gas filled, voids by diffusion bonding. However, on the debit side, some mechanical properties of cast parts, particularly those which are crack initiation-related, such as smooth fatigue, are currently inferior to those exhibited by ingot metallurgy (IM) parts.
The melting practice used for cast-part production is essentially the same as for alloy ingot melting. Accordingly, it is possible to cast all titanium alloys produced by ingot metallurgy. The major difference between ingot metallurgy and cast metallurgy parts stems from the subsequent hot working and heat treatment of ingots or their products, which allows microstructural manipulations not possible in the cast part, such as, for example, equiaxed recrystallized alpha.
Smickley et al, U.S. Pat. No. 4,505,764 (Mar. 19, 1985) disclose treatment of the microstructure of titanium alloy castings which comprises the steps of heating the casting to a treatment temperature of about 800.degree. to 2000.degree. F., the treatment temperature being below the beta transus temperature of the alloy, diffusing hydrogen into the casting at treatment temperature such that hydrogen is present in an amount ranging from 0.2 to 5.0 wt. percent, and removing the hydrogen. The method of Smickley et al requires maintaining the temperature of the casting above the temperature at which metal hydrides would be formed when hydrogen is present in the casting in more than trace amounts. Smickley et al disclose that cooling the hydrogenated casting to about room temperature wherein significant amounts of titanium hydride could form, results in cracking and distortion of the casting. A major drawback of the method of Smickley et al is the requirement for a relatively sophisticated apparatus, capable of performing both hydrogenation and dehydrogenation.
Levin et al, U.S. Pat. No. 4,612,066 (Sept. 16, 1986) disclose treatment of the microstructure of titanium alloy castings which comprises the steps of beta-solution heat treating the casting, rapidly cooling the casting to room temperature, hydrogenating the casting at a temperature below the beta-transus and dehydrogenating the casting. The beta-solution heat treatment followed by rapid cooling can lead to component cracking or distortion.
Hydrogen has also been used to increase the high temperature ductility of titanium alloys. Lederich et al, U.S. Pat. No. 4,415,375 (Nov. 15, 1983) disclose a method for superplastically forming titanium and titanium alloys which comprises treating a stock piece of titanium or titanium alloy with hydrogen to form a transient alloy containing hydrogen, superplastically forming the hydrogen containing piece, and thereafter, removing the hydrogen from the formed piece.
Zwicker et al, U.S. Pat. No. 2,892,742 (June 30, 1959) disclose a process for hot working of titanium alloys which comprises incorporating about 0.05 to 1 weight percent of hydrogen into such alloys, hot working the hydrogen-containing alloys, and removing the hydrogen therefrom after the hot working has been completed.
Although Zwicker et al and Lederich et al have disclosed that hydrogen is beneficial as a transient alloying element for improving the hot workability and superplasticity of titanium and its alloys, pure titanium and many titanium alloys are embrittled at room temperature by the presence therein of only very small quantities of hydrogen. This embrittlement causes a lowered impact resistance. In order to obtain good mechanical properties at room temperature, it is necessary to remove the hydrogen therefrom after hot working or superplastic forming has been completed.
Further, the improved hot workability of titanium alloys containing hydrogen does not extend to alloys which are temporarily alloyed with hydrogen, then dehydrogenated under vacuum prior to hot forging. W. R. Kerr et al, "Hydrogen as an Alloying Element in Titanium (Hydrovac)". Titanium '80 Science and Technology, (1980) pp 2477-2486.
It is an object of this invention to provide a method for improving the microstructure of cast titanium alloy articles.
Other objects and advantages of the present invention will be apparent to these skilled in the art from a reading of the following detailed description of the invention.